Aptian boundary interval in the Tethys Realm, Mexico and Romania

Cretaceous Research 27 (2006) 529e541 www.elsevier.com/locate/CretRes

Palaeoenvironmental and palaeobiologic changes across the Barremian/Aptian boundary interval in the Tethys Realm, Mexico and Romania Ricardo Barraga´n a,*, Mihaela Carmen Melinte b b

a Instituto de Geologı´a, UNAM, Departamento de Paleontologı´a, Ciudad Universitaria, 04510 Me´xico, DF, Mexico National Institute of Marine Geology and Geoecology (GeoEcoMar), Dimitrie Onciul 23e25, sector 2, 70318, Bucharest, Romania

Received 21 June 2004; accepted in revised form 6 October 2005 Available online 12 June 2006

Abstract Upper Barremianelower Aptian rock sequences in northeast Mexico and the South Carpathians of Romania display palaeoenvironmental and palaeobiologic similarities. The upper Barremian deposits in both areas studied accumulated in shallow marine environments. A latest Barremian transgression was recorded only in Romania, where it is characterized by the prevalence of marly sequences that include mixed Tethyan and Boreal floras and faunas. In both study areas, late Barremian shallow-marine carbonate platforms were succeeded by deep-marine facies in the early Aptian. This was probably related to the early Aptian global sea-level rise. The Barremian/Aptian boundary in both areas is indicated approximately by the first occurrence of the foraminifer species Palorbitolina lenticularis, a bioevent that is coincident with the acme of the foraminifer group Orbitolina. In the early Aptian, Tethyan faunas and floras characteristic of the late Barremian were replaced by cosmopolitan assemblages, an event coincident with the first appearance in Romania of the ammonite genus Deshayesites. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Barremian/Aptian boundary; Palaeoenvironmental changes; Palaeobiological turnover; Mexico; Romania

1. Introduction The transition interval of the Barremian/Aptian boundary marks the beginning of mid-Cretaceous geological changes that have been recorded on a global scale. Extensive intraplate volcanism accompanied by increased ocean-floor spreading (Larson, 1991; Tatsumi et al., 1998) caused increased atmospheric CO2; as a result the earth’s climate became warm and humid. These mid-Cretaceous (AptianeTuronian) greenhouse conditions were accompanied by low latitudinal temperature gradients, high humidity and sluggish oceanic circulation (Barron, 1983; Hay, 1995). Perturbations that affected the oceans at that time are mirrored in lithological changes. The

* Corresponding author. E-mail address: [email protected] (R. Barraga´n). 0195-6671/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2005.10.016

Lower Cretaceous reefal limestones of the Tethyan realm were replaced by predominantly marly sequences around the Barremian/Aptian boundary. The widespread deposition of the Livello Selli (Oceanic Anoxic Event 1a) in many Tethyan areas (Arthur and Premoli-Silva, 1982; Coccioni et al., 1989; Bralower et al., 1994) may be regarded as a response to the major igneous events of the Barremian/Aptian boundary interval (Larson and Erba, 1999). The palaeophysiographic changes were accompanied by a biotic turnover (Mutterlose, 1992; Erba, 1994; Aguado et al., 1997; Bischoff and Mutterlose, 1998; Mutterlose, 1998) reflected in marine floral and faunal fossil assemblages. Marine organisms (e.g., ammonites, calcareous nannofossils) that displayed a considerable degree of provincialism during the Early Cretaceous were replaced by more cosmopolitan taxa. To test the hypothesis of global palaeoenvironmental and palaeobiological changes in the transition interval of the

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Barremian/Aptian boundary, two areas that are far apart in the Tethyan realm, northeast Mexico and the Southern Carpathians of Romania, were selected for study (Fig. 1). In spite of the distance separating these regions, and their different tectonic and physiographic evolution, their respective geologic records appeared to be similar. The aim of our study was to determine whether similar patterns of change could be identified in the lithologic, sedimentologic and/or palaeontologic records of the Barremian/Aptian boundary interval. We also wanted to find out whether these changes could be related to regional and/or global forcing factors.

2. Geological setting The Barremian/Aptian boundary transition in Mexico is characterized by continuous marine sedimentation in two tectonic-sedimentary regimes: (1) flysch-like sedimentation associated with the activity of a volcanic island-arc in the southwest of the country, and (2) normal marine sedimentation related to the development of a passive margin in the northeast, which also includes the best outcrops with a complete record of the sedimentologic and palaeontologic

changes that occurred through this interval. The stratigraphic sequence discussed in this paper is located at the Francisco Zarco Dam in Durango State, along the western exposed limb of an open anticline at the southern end of the Sierra del Rosario (Fig. 1). It is part of the terranes that were involved in the Laramide Orogeny, which controlled the complex sedimentary structures of the folded Mexican cordillera (Mora´n-Zenteno, 1994). The Carpathian regions of Romania also show that marine sedimentation was continuous across the Barremian/Aptian boundary. Pelagic facies occur in the South Carpathians, whereas the East Carpathians deposits are mainly turbiditic. One of the best exposures of the interval studied is in the Muierii valley of the Daˆmbovicioara area at the eastern end of the South Carpathians, where the sedimentary sequences are part of the Getic Nappe (sensu Sandulescu, 1984). The Lower Cretaceous deposits of the region are referable to the Daˆmbovicioara Formation, which is divided into two members: a lower Dealul Sasului Member of BerriasianeHauterivian age and an upper Valea Muierii Member of Barremiane early Aptian age (Patrulius and Avram, 1976). The section considered herein is the stratotype of the upper part of the formation.

N North America

Europe

Asia

Africa South America Australia

MEXICO

ROMANIA s thian arpa

y ar

30° N

Durango Se

rbi

a

i en us ns Ap ntai u Mo

C East

Hu

ng

USA

Ukraine

48° N

f co bli ia pu av Re old M

90° W

rpathian South Ca

Ukraine

Study area

s

c ifi

ac

P

Bucharest

O

Mexico City

Danub

e

n

a ce

115° W

44° N 15° N

20° E

Fig. 1. Location of the BarremianeAptian sections studied in Mexico and Romania.

Black Sea

a

Bulgari

28° E

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3. Material and methods

4. Results

Our stratigraphic analysis focused on detailed lithologic, palaeontologic, and petrographic characteristics of the different facies. Several field seasons allowed us to complete detailed observations of variations in individual strata, as displayed in Figs. 2 and 3. Field identification of initial formational characteristics was based on lithostratigraphic units established by the geologists cited below. Samples were taken from each layer and thin sections prepared for characterization of their microfacies. We placed special emphasis on every palaeontologic and petrographic component that allowed us to further characterize the vertical distribution of different types of microfacies and subdivide the formations into informal subunits (Figs. 2, 3). Micropalaeontologic assemblages were used as the main criteria in the Mexican section to differentiate clusters of strata that constitute informal Units 1e7 (Fig. 2), whereas ammonites were used for informal Unit 8. Identification of the microfossils followed the taxonomic guidelines of Hottinger (1967) and Neumann (1967). The ammonite records of Unit 8 (Fig. 2) have been particularly helpful as individual morphotypes consist of well-preserved internal moulds and casts that allowed identification of taxa to genus and species. The ammonite data thus yielded sufficient index taxa to identify a standard lower Aptian biozone that characterizes the Tethyan Realm (Hoedemaeker and Reboulet et al., 2003). In the Romanian section, samples were taken at 10-cm intervals for calcareous nannoplankton analysis. Preparation of the smear-slides followed the method described by Lamolda et al. (1994). The slides were studied under a polarizing microscope at 1600 magnification. Preservation of the nannoplankton proved to be fair in lithological Units 1, 2 and 4 and good in Units 3 and 5. Qualitative identification of taxa was complemented by a quantitative analysis based on a count of 300 specimens per smear-slide in randomly distributed longitudinal transverses and involved essentially six taxonomic groups: (1) Watznaueria barnesae, an extremely cosmopolitan species resistant to diagenesis; (2) Tethyan Nannoconus species, oligotrophic taxa that are indicative of warm surface waters (Erba, 1994; Melinte and Mutterlose, 2001); (3) Assipetra, including A. terebrodentarius and A. infracretacea, taxa more confined to lowemiddle latitudes (Aguado et al., 1997; Tremolada and Erba, 2002; Herrle and Mutterlose, 2003); (4) Micrantholithus, including M. hoschulzii and M. obtusus, together with Braarudosphaera (B. regularis, B. batiliformis and B. hockwoldensis), taxa probably suggestive of low-salinity (Bersezio et al., 2002) and warm surface (Melinte and Mutterlose, 2001) waters; (5) Zygodiscus erectus and Cyclagelosphaera margerelii, taxa indicative of a more eutrophic environment (Erba, 1994; Premoli Silva et al., 1999); and (6) Boreal taxa (sensu Mutterlose, 1996) including Sollasites horticus, Crucibiscutum salebrosum, Zeugrhabdothus sisyphus and Vagalapilla matalosa. The relative abundance of each of these groups of taxa was calculated as a percentage of the total count, which represents more than 90% of all nannofloras, and diversity is the number of identified taxa.

4.1. Lithostratigraphy

531

4.1.1. Mexico The section studied in Mexico consists of marine carbonate facies of the upper part of the Cupido Formation (Imlay, 1937; Humphrey, 1949) of Barremianeearliest Aptian age, and of the base of the La Pen˜a Formation (Imlay, 1936; Humphrey, 1949), which was deposited during the late earlyelate Aptian. It can be subdivided into eight lithological units, 1e7 encompassing the top of the Cupido Formation, and 8 comprising the base of the La Pen˜a Formation. The microfacies of these units are described in ascending order as follows (Fig. 2): Unit 1: 18 m of biocalcilutites in beds 25e100 cm thick, rich in benthonic foraminifera, particularly of the miliolid group. Unit 2: 13 m of extensively bioturbated pelletoidal calcarenites in beds 80e200 cm thick containing common ostracods and diverse benthonic foraminifera that include taxa with arenaceous tests and miliolids. Unit 3: 4-m-thick biocalcarenite with sparry calcite cement, and allochems of oolites, peloids and green algae. Unit 4: 10 m of matrix-supported biocalcirudites in beds 80e 400 cm thick containing randomly orientated rudist fragments and benthonic foraminifera of the orbitolinid group. On the basis of on palaeontological data (Barraga´n-Manzo and Dı´az-Otero, 2004 and herein) the Barremian/Aptian boundary is placed at the base of this unit. Unit 5: 2 m of grain-supported biocalcarenites with pellets, and mollusc and echinoid fragments. Unit 6: 6-m-thick bed of biocalcilutite with common benthonic foraminifera of the orbitolinid group. Unit 7: 8.5 m of biocalcarenites in beds 97e250 cm thick, very rich in benthonic foraminifera of the miliolid group as well as common intraclasts and mollusc fragments. Unit 8: 23 m of alternating biocalcilutites and marls in beds 10e40 cm thick, with abundant planktonic foraminifera, common ostracods, and echinoid fragments. This unit is very rich in ammonoids. It can be subdivided into three distinct intervals based on sedimentological composition: (1) the lowest 8 m are marked by the sudden disappearance of the benthic components along with an increase in total organic matter; (2) the succeeding 10 m are characterized by low concentrations of organic carbon together with a slight recovery in the relative abundance of the benthonic foraminifera; (3) the top 5 m display distinctive chert horizons with high concentrations of organic matter associated with abundant radiolaria (Barragan, 2001). 4.1.2. Romania The Romanian section is composed of marine carbonates of the Valea Muierii Member, the upper member of the Daˆmbovicioara Formation. The Barremian/Aptian boundary interval

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Unit 3

PFZ-45c PFZ-45a

Unit 2

P U

M E C

PFZ-43c

Unit 1

PFZ-40b PFZ-40a

PFZ-34d

B

A

R

R

PFZ-44c

20m

0m

Legend

calcilutites

Common Orbitolinidae

Common Radiolaria

Palorbitolina lenticularis and Pseudocyclammina hedbergi

Common benthonic Foraminifera

Common planktonic Foraminifera

Kazanskyella arizonica

PFZ-47a

?

Pseudocyclammina litus

40m

Common Miliolidae, Lituolidae, Valvulinidae, Textularidae, Nezzazatidae and Ataxophragmiidae

PFZ-53 PFZ-51b PFZ-49 PFZ-48 PFZ-47b

Rhytidoplites robertsi and Burckhardtites nazasensis

Paracymatoceras milleri

PFZ-61c PFZ-61b PFZ-61a PFZ-58c PFZ-57c PFZ-56

Peltocrioceras spp. Acrioceras (Epacrioceras) spp.

Unit 7 Unit 4

(Barragán and Díaz, 2004)

Caprinid rudists

FORMATION

I

MICROFAUNAS

(Barragán, this work)

PFZ-55a

D

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U 5 Unit 6

60m

MACROFAUNAS Colombiceras spathi

Unit 8

Dufrenoyia justinae and D. spp.

80m

L A

P E Ñ A

LITHOLOGY

I

A

N

L O W E R

A P T I A N STAGE

532

PFZ-29a

marly limestones

pelletoidal calcarenites

oolithic calcarenites

rudistic calcirudites

Fig. 2. Lithology and biostratigraphy of the Upper BarremianeLower Aptian deposits of northeast Mexico. Samples PFZ-29ae61c were taken from strata of the Cupido Formation. Units 1e8 are informal subdivisions based on temporal variations in microfacies textures.

includes five lithologic units (Fig. 3) that can be differentiated in ascending order as follows: Unit 1: 15 m of white bioclastic calcarenites and calcilutites, rich in benthonic foraminifera of the miliolid group. Unit 2: 7.5 m of alternating grey marls and calcarenites in beds 20e40 cm thick, with thin intercalations of bioclastic calcarenite. Unit 3: 10 m of very light grey marls. Unit 4: 26 m of alternating marls and calcarenites in beds 10e30 cm thick. This unit contains rudist and mollusc fragments as well as thin intercalations of bioclastic calcarenite similar to that in Unit 2. The base of Unit 4 includes a 1-mthick calcarenite bed rich in orbitolinids, and is coincident with the Barremian/Aptian boundary as determined from palaeontological data (Neagu, 1975; Patrulius and Avram, 1976; Melinte, 2002). Unit 5: 9 m of bluish grey marls.

4.2. Biostratigraphy 4.2.1. Mexico The upper Barremian biocalcilutites and biocalcarenites (Units 1e3 of the Sierra del Rosario section) are characterized by a rich microfossil assemblage dominated by benthonic foraminifera referable to at least five families, namely: Ataxophragmiidae, Lituolidae, Textularidae, Nezzazatidae, Valvulinidae, and Miliolidae (Fig. 2). Important biostratigraphic indicators within these units are: Pseudocyclammina litus (Yokokoyama), Pseudotextulariella scarsellai (de Castro), Chofatella decipiens Schlumberger, Vercorsella arenata Arnaud-Vanneau, Debarina hahounerensis Fourcade, Thaumatoporella parvovesiculifera Raineri, Bacinella irregularis Radoicic, Cuneolina spp., Glomospira spp., Nezzazata spp., and Everticyclammina sp. Overlying Units 4e7 are dominated by the same families of benthonic foraminifera but a major faunal change occurs at the base of Unit 4, namely the appearance of taxa representing the

MICROFAUNAS (Neagu, 1975)

A B

Phyllopachyceras stuckenbergi

Unit 1

P U Legend

0m

calcilutites

calcarenites

reefal calcarenite

NANNOFOSSILS EVENTS

ZONES

Beginning of the nannoconid “crisis”

Conusphaera mexicana

Braarudosphaera hockwoldensis Braarudosphaera batiliformis

Chiastozygus litterarius

Costidiscus recticostatus

Barremites strettostoma

Silesites seranonis

Palorbitolina lenticularis

P

E

R

10m

Lithancyclus spp.

20m

Heteroceras spp.

30m

Parancyloceras meridionale

Unit 4 40m

D A M B O V I C I O A R A Unit 2 Unit 3

F O R M T I O N

50m

R

R

E

M

I

A

N

L

O

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E

R

A

P

T

I

A

Neohibolites spp.

60m

533

FO of boreal taxa (Crucibiscutum salebrosum, Zeugrhabdothus sisyphus, Sollasites horticus)

Hayesites irregularis

Vagalapilla matalosa

Flabellites oblongus Chiastozygus litterarius Conusphaera rothii Rhagodiscus pseudoangustus

Micrantolithus hoschulzii

Pseudohaploceras douvillei Audouliceras thomeli Deshayesites spp.

acme

N

Unit 5

67m

MACROFAUNAS (Patrulius and Avram, 1976; Avram and Melinte, 1998)

Acme of Epistomina and common Dorothia spp., Spiroplectammina spp. and Nodosaria spp.

LITHOLOGY

Dominance of Lenticulina , Gaudryina, Common Miliolidae, Lituolidae and Valvulinidae Uvigerinammina, Pseudocyclammina genera Orbitolina

STAGE

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calcarenite rich in orbitolinids

marls

Fig. 3. Lithology and biostratigraphy of the upper Barremianelower Aptian deposits in the South Carpathians of Romania.

family Orbitolinidae, and caprinid rudists. The Barremian/ Aptian boundary was recently placed within the unit (Barraga´n-Manzo and Dı´az-Otero, 2004) based on the first appearance of Palorbitolina lenticularis (Blumenbach) and the last occurrence of Pseudocyclammina litus (Neumann) (Fig. 2), which are synchronous. The four units are also characterized by the appearance of typical Aptian forms, such as Pseudocyclammina hedbergi Maync. In Table 1 we show the

different strata studied in Units 1e7 and their respective micropalaeontological assemblages, which allow us to infer their stratigraphic ranges. Unit 8 is very rich in ammonites with clear Tethyan affinity (Fig. 2). Its base is characterized by an assemblage with abundant specimens of the typical American index taxon Dufrenoyia justinae (Hill), indicative of the early Aptian. The unit also includes D. boesei Humphrey, D. durangensis Humphrey,

534

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Table 1 Micropalaeontological assemblages and their possible stratigraphic ranges through each bed of the Cupido Formation in Mexico SAMPLE PFZ-29a PFZ-30c PFZ-34a PFZ-34b PFZ-34d PFZ-40a PFZ-40b PFZ-42 PFZ-43b PFZ-44b PFZ-47a PFZ-47b PFZ-48 PFZ-49 PFZ-57c PFZ-58b PFZ-58c PFZ-59b PFZ-61a PFZ-61b PFZ-61c

MICROPALAEONTOLOGICAL ASSEMBLAGE Glomospira sp., Cuneolina sp., Favreina sp., Thaumatoporella parvovesiculifera, Nezzazatinae, Lituolidae, echinoid spines, ostracods, mollusc fragments. Nezzazatinae, Glomospira sp., echinoid spines, mollusc fragments. Nezzazatinae, Glomospira sp., Ophthalmidium sp., Miliolidae, Valvulinidae, mollusc fragments. Cuneolina sp., Glomospira sp., Ataxophragmiidae, Ammodiscidae, Miliolidae, Textularidae, calcareous algae, mollusc fragments, echinoid spines. Pseudocyclammina sp., Glomospira sp., Vercorsella cf. arenata, Miliolidae, Nezzazatinae. Chofatella decipiens, Pseudocyclammina litus, Glomospira sp., Nezzazatinae, Miliolidae, Textularidae, mollusc fragments, ostracods. Chofatella decipiens, Pseudocyclammina litus, Debarina sp., Glomospira sp., Cuneolina sp., Nezzazatinae, Miliolidae, mollusc fragments. Glomospira sp., Nezzazatinae, Textularidae, Lituolidae, Miliolidae, mollusc fragments. Pseudotextulariella cf. scarsellai, Glomospira sp., Pseudocyclammina litus, Salpingoporella sp., Miliolidae, ostracods. Salpingoporella aff. annulata, Chofatella decipiens, Bacinella irregularis, Pseudocyclammina litus, Miliolidae, ostracods, mollusc fragments, echinoid spines. Pseudocyclammina litus, Debarina cf. hahounerensis, Glomospira sp., Salpingoporella sp. Palorbitolina cf. lenticularis, Vercorsella cf. arenata, Chofatella decipiens, Pseudocyclammina hedbergi, Eggerella sp., Debarina sp., Glomospira sp. Miliolidae, mollusc fragments, echinoid spines. Pseudocyclammina hedbergi, Glomospira sp., Vercorsella cf. arenata, Palorbitolina cf. lenticularis, Pseudocyclammina sp., Everticyclammina sp., Salpingoporella cf. Dinarica, Nezzazatinae. Salpingoporella cf. annulata, Glomospira sp., Pseudocyclammina sp., Cuneolina sp., Debarina aff. hahounerensis, Palorbitolina cf. lenticularis, Nautiloculina sp., Nezzazatinae, Miliolidae, echinoid spines. Everticyclammina sp., Valvulammina sp., Debarina cf. hahounerensis, Palorbitolina cf. lenticularis, Pseudocyclammina sp., Nezzazatinae, Miliolidae, algae. Valvulammina sp., Salpingoporella sp., Palorbitolina cf. lenticularis, Thaumatoporella parvovesiculifera, Nezzazatinae, Miliolidae. Bacinella irregularis, Pseudocyclammina hedbergi, Nezzazatinae, Valvulinidae, mollusc fragments. Cuneolina sp., Vercorsella sp., Glomospira sp., Palorbitolina cf. lenticularis, Textularia sp., Nezzazatinae, mollusc fragments, echinoid spines. Chofatella decipiens, Pseudocyclammina hedbergi, Cuneolina sp., Salpingoporella sp., Palorbitolina cf. lenticularis, Valvulina sp., Textularia sp., Nezzazatinae, Miliolidae. Pseudocyclammina hedbergi, Pseudocyclammina sp., Glomospira sp., Nezzazatinae, algae. Chofatella decipiens, Debarina sp., Glomospira sp., Eggerella sp., Palorbitolina cf. lenticularis, Nezzazatinae, Miliolidae, Textularidae.

STRATIGRAPHIC RANGE Neocomian upper Barremian–lower Turonian upper Barremian–lower Turonian Indefined upper Barremian–lower Aptian Barremian upper Barremian–lower Aptian upper Barremian–lower Turonian Hauterivian–Barremian Neocomian–lower Aptian Hauterivian–lower Aptian lower Aptian lower Aptian upper Barremian–lower Aptian upper Barremian–lower Aptian lower Aptian upper Barremian–lower Aptian upper Barremian–lower Aptian upper Barremian–lower Aptian upper Barremian–lower Aptian lower Aptian

The shaded area encompassing sample PFZ- 47b is locally referred to as the Barremian/Aptian boundary.

D. dufrenoyi (d’Orbigny), and D. scotti Humphrey. Among other cephalopods that characterize the La Pen˜a Formation at the locality studied, Colombiceras spathi Humphrey and Paracymatoceras milleri Humphrey occur in the lower part of the unit. They are succeeded by such species as Burckhardtites nazasensis (Burckhardt) and Rhytidoplites robertsi Scott (Fig. 2), which become dominant, while other concurrent taxa including Kazanskyella arizonica Stoyanow, Peltocrioceras spp., and Acrioceras (Epacrioceras) spp. remain in lower numbers. The ammonite assemblage contains taxa that are characteristic of the local Dufrenoyia justinae Zone (Barraga´n-Manzo and Me´ndez-Franco, 2005), which is coeval with the Tethyan Dufrenoyia furcata Zone of the standard biozonation for the Mediterranean Region (Hoedemaeker and Reboulet et al., 2003). 4.2.2. Romania Upper Barremian calcarenites and calcirudites that define Unit 1 of the Romanian section are characterized by ammonite assemblages of Tethyan origin (Fig. 3). These are dominated by species of the genera Lithancyclus and Heteroceras (Patrulius and Avram, 1976), and also include Barremites strettostoma (Uhlig) and Phyllopachyceras stuckenbergi (Karakasch). Calcareous nannoplankton assemblages associated with these rocks are likewise dominated by Tethyan taxa. Genera such

as Nannoconus and Conusphaera, together with Assipetra terebrodentarius (Applegate et al., in Covington and Wise) Rutledge and Bergen, in Bergen, represent 60e70% of the nannofloras (Fig. 4), whereas Assipetra spp. constitute 1.5e7% of the assemblages, and show an overall inverse correlation with Nannoconus. The most frequently recorded cosmopolitan nannofossils belong to the genera Watznaueria, Zeugrhabdothus and Micrantholithus. The abundance of Watznaueria barnesae (Black) Perch-Nielsen fluctuates between 12 and 24% of the assemblages, and shows an inverse correlation with Nannoconus, but its fluctuations are synchronous with those of Micrantholithus and Braarudosphaera (Fig. 4). Also noteworthy is the abundance of Zygodiscus erectus and Cyclagelosphaera margerelii, which varies between 0.5 and 2%. The following nannofossil events were recorded in Unit 1: LO (last occurrence) of Rhagodiscus pseudoangustus Crux, succeeded by the LO of Conusphaera rothii (Thierstein) Jakubowski, and subsequent successive FOs (first occurrences) of Chiastozygus litterarius (Go´rka) Manivit and Flabellites oblongus (Bukry) Crux (Fig. 3). As shown in Fig. 4, nannoflora diversity varies little, staying at around 20 taxa. The foraminiferal assemblages contain common Valvulinidae and Miliolidae, but the most frequent taxa are Lingulogavellina barremiana,

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Chofattella decipiens, Pseudocyclammina cylindrica, Spiroplectammina schultzei, Valvulammina rotaliformae, Lenticulina spp. and Nodosaria spp. (Neagu, 1975). Unit 2 is also characterized by ammonite assemblages of only Tethyan taxa, with Lithancyclus and Heteroceras still common (Avram and Melinte, 1998), together with Silesites seranonis (d’Orbigny), Eulytoceras phestum (Matheron), Costidiscus recticostatus (d’Orbigny), Barremites strettostoma and Phyllopachyceras stuckenbergi. Similarly, the calcareous nannofloras are dominated by Tethyan taxa referable to Nannoconus, including N. steinmannii Kamptner, N. colomii (de Lapparent) Kampter, N. kamptneri Bro¨nnimann, N. vocontiensis Deres and Ache´rite´guy, N. minutus Bro¨nnimann, N. bucheri Bro¨nnimann, and N. circularis Deres and Ache´rite´guy, as well as Conusphaera mexicana Trejo. The abundance of Tethyan taxa decreases from over 60% close to the base of Unit 2 to 50% towards the top (Fig. 4), whereas Watznaueria barnesae reaches a maximum frequency of 30% close to the top. Among the significant taxa, Assipetra spp. remain within the range of 2e6%, while species of Micrantholithus and Braarudosphaera increase up to 15%, and Zygodiscus erectus along with Cyclagelosphaera margerelii fluctuate between 2 and 5%. Nannofloral diversity in Unit 2 is almost constant at about 22 taxa (Fig. 4), and there is no apparent change in the benthonic foraminiferal assemblages from Unit 1. Unit 3 also contains diverse Tethyan ammonite assemblages (Avram and Melinte, 1998), including Silesites seranonis, Eulytoceras phestum, Costidiscus cf. recticostatus, Barremites strettostoma, Heteroceras spp., Protetragonites crebisulcatus (Uhlig), and Eotetragonites ex gr. duvalianus (d’Orbigny). The last occurrence of Phyllopachyceras stuckenbergi is recorded at the base of the unit (Fig. 3). Specimens of the Boreal species Parancyloceras meridionale (Avram) occur throughout. Nannofloral assemblages remain of Tethyan character, similar to those of Units 1 and 2. Again, over 50% of the total nannoflora is represented by Nannoconus spp. and Conusphaera mexicana, which show very slight fluctuations in abundance. Vagalapilla matalosa (Stover) Thierstein, Hayesites irregularis (Thierstein, in Roth and Thierstein) Covington and Wise and Braarudosphaera batiliformis Troelsen and Quadrio have their respective successive FOs within the unit (Fig. 3). In addition to Tethyan and cosmopolitan nannofossils, a small group of Boreal taxa (sensu Mutterlose, 1996) occur in the succession slightly above the FO of Vagalapilla matalosa. This group includes Zeugrhabdothus sisyphus (Gartner) Crux, Sollasites horticus (Stadner et al., in Stadner and Adamiker) Cepek and Hay and Crucibiscutum salebrosum (Black) Jakubowski, which together with Vagalapilla matalosa, reaches a peak abundance of 15% in the middle part of the unit. On the other hand, whereas Watznaueria barnesae decreases to around 20% from the base to the top of Unit 3, strong fluctuations shown by Assipetra spp., albeit of significantly lower percentages overall (Fig. 4), do not seem to follow any pattern in relation to the other taxa. The group represented by Micrantholithus and Braarudosphaera decreases in abundance up-section while Zygodiscus erectus and Cyclagelosphaera margerelii fluctuate between 0.5 and 3.5%. The overall

535

nannofloral diversity averages 30 taxa. The three benthic foraminiferal assemblages (Neagu, 1975) are different from those in Unit 2 in being dominated by species of such genera as Lenticulina, Gaudryina, Nodosaria, Uvigerinammina and Pseudocyclammina (Fig. 3). The base of overlying Unit 4 contrasts with the predominately marly Unit 3 in consisting of 1 m of calcarenite rich in orbitolinid foraminifera. This base of this unit is considered to be the Barremian/Aptian boundary, which we regard as approximately coinciding with the FO of Palorbitolina lenticularis, as in Mexico (Fig. 2). The basal calcarenite of Unit 4 is poor in ammonites, but it contains Eulytoceras phestum and Costidiscus costatus (Avram and Melinte, 1998). Its nannoflora assemblage is diverse (28 taxa), consisting of over 75% Tethyan taxa, with Nannoconus reaching up to 55%. The boreal group that peaked in the middle part of Unit 3 is poorly represented, being as low as 1% in the calcarenite bed, whereas Watznaueria barnesae increases up to 30%. The abundance of Assipetra spp. at this level is around 4%, while representatives of Micrantholithus and Braarudosphaera are a little more common, and Zygodiscus erectus together with Cyclagelosphaera margerelii comprise less than 1% (Fig. 4). The calcarenite bed also contains the FO of Braarudosphaera hockwoldensis Black. The macrofauna of the lowermost Aptian above the calcarenite bed (Patrulius and Avram, 1976; Avram and Melinte, 1998) consists of entirely Tethyan ammonites: Eulytoceras phestum, Costidiscus recticostatus, Barremites strettostoma, Protetragonites crebisulcatus, Eotetragonites duvalianus, Pseudohaploceras douvillei (Fallot), Argvethites godoganiensis Eristavi, Audouliceras thomeli Avram, and the LO of Silesites seranonis. Immediately above the orbitolinid-rich bed, Deshayesites is absent, but the FO of Neohibolites (N. ewaldi Stombeck) is recorded at the top of the unit (Fig. 3). The foraminiferal assemblages are typified by the dominance of species of Epistomina (Neagu, 1975). The nannofloras consist mainly of a mixture of cosmopolitan and Tethyan morphotypes; their diversity declines progressively, the average number of taxa being 25. Watznaueria barnesae is abundant, increasing up to 39% towards the top of the unit. Other common cosmopolitan taxa include Zeugrhabdothus embergeri, Assipetra infracretacea and Braarudosphaera spp. (B. batiliformis, B. bigelowii and B. hockwoldensis). Although Nannoconus spp. decline, they still represents an important component of the nannofloras, averaging 35%. Numbers of Boreal taxa again increase (up to 7%) toward the top of the unit (Fig. 4), along with increasing percentages of several other genera and species, including Assipetra, Micrantholithus, Braarudosphaera, Zygodiscus erectus and Cyclagelosphaera margerelii. The uppermost part of the section studied, Unit 5 (lower Aptian), contains the first occurrence of the ammonite Deshayesites (Deshayesites deshayesi). Calcareous nannofossils in this unit are dominated by cosmopolitan species, which represent 60e70% of the nannofloral assemblages. The abundance of Tethyan species is low (10e20%) by comparison with the older intervals. Nannoconus spp. show an abrupt

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decline to less than 10% towards the top of the unit, a bioevent that also coincides with a pronounced decline in diversity from 22 to 12 species. Furthermore, Unit 5 is characterized by large numbers of Watznaueria barnesae (almost 40%), large fluctuations of Assipetra spp., and an increasing abundance of Micrantholithus and Braarudosphaera (up to 15%). Boreal nannofossils also increase substantially, reaching a maximum abundance of 10e15%. A similar sharp increase in abundance is shown by Zygodiscus erectus and Cyclagelosphaera margerelii, these taxa together reaching 7e8% at the top of the unit. The foraminiferal assemblages are dominated by species of Epistomina (Neagu, 1975), together with common Dorothia, Spiroplectammina and Nodosaria. 5. Discussion 5.1. Palaeoenvironment The composition and general characteristics of upper Barremianelowermost Aptian facies of the Cupido Formation in Durango indicate deposition within a shallow lagoonal environment of a carbonate platform. Previous studies have recognized that extensive carbonate platform buildup dominated the palaeogeography of northeast Mexico during this time

interval (Bo¨se, 1923; Burckhardt, 1930; Imlay, 1936, 1937, 1938, 1944; Kellum, 1944). The prevailing abundance of miliolid and arenaceous foraminifera in Units 1 and 2 (Fig. 2) suggests periods of relatively restricted circulation within a protected environment, whereas the oolitic limestones with green algae in Unit 3 represent periods of more oxygenated, shallower waters. Facies of Units 1e7 assigned to the Cupido Formation in the section studied are similar to the unit described by Conklin and Moore (1977) as ‘‘Cupidito’’, which was interpreted to represent subtidal sedimentation within a lagoon that was restricted by a barrier reef composed of rudists. A barrier reef nearby is also inferred from the composition of Unit 4 in the Durango section. Subtidal sedimentation is implied by the textural composition of Unit 2, which is characterized by extensively bioturbated pelletoidal calcarenites with common ostracods, grading upward to ooid-peloidal biocalcarenites (Unit 3), and biocalcirudites with rudists (Unit 4). The nature of the remaining lithologic units (5e7) within the Cupido Formation, which are mainly composed of miliolids, orbitolinids, pellets, and mollusc and echinoid fragments, further suggests that the depositional processes that led to these facies correspond to a lower energy, restricted lagoonal environment. The onset of deposition of Unit 8, or the La Pen˜a Formation (Fig. 2), represents a shift from a set of facies indicative of

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a shallow carbonate platform to a facies representing a more open-marine environment typical of an outer ramp or slope. This major change in the depositional regime is explained by the initiation of a major transgressive event that would eventually be one of the main factors controlling the drowning of the carbonate platform. The sudden disappearance of benthic foraminifera and the proliferation of planktonic foraminifera in the lowermost part of Unit 8 are interpreted to represent deposits originating in stagnant bottom waters during the initial stage of the transgression. This assertion is further supported by the coeval abundance of organic matter in this part of the unit (Barragan, 1999, 2001). Subsequent low concentrations of organic carbon in the middle part of the unit, together with a slight recovery in the relative abundance of benthic foraminifera, indicate a period in which oxic bottom conditions were reestablished prior to complete drowning of the carbonate platform. Deposition of the upper part of Unit 8 is characterized by the sudden appearance of radiolaria, which became dominant over the planktonic foraminifera. The populations of benthic foraminifera decline again until they disappear towards the top of the unit (Barragan, 1999, 2001). Radiolaria have been identified as important constituents of Recent and ancient upwelling systems (Erbacher and Thurow, 1997). Funnell (1987) indicated that high concentrations of organic carbon in sediments deposited under upwelling systems are the result of increased productivity and burial rates of organic matter owing to eutrophic surface waters. In such environments, benthic forms normally disappear whereas shallow dwellers survive. Similarly, the top of Unit 8 is interpreted here as having been deposited under anoxic bottom conditions related to eutrophication of surface waters. These resulted from a local upwelling system in a basinal environment, and represent the final stage of the drowning process of the BarremianeAptian platform in northeast Mexico (Barragan, 1999, 2001). Similarly, upper Barremian reefal bioclastic calcarenites and calcirudites with fragments of rudists and molluscs recorded in the Romanian section are indicative of a shallow-marine reef environment on a carbonate platform, which covered the South Carpathian area during most of the Early Cretaceous. The high percentages of miliolids in Units 1 and 2 suggest that water circulation was restricted in this region during the late Barremian. Furthermore, the abundance of the Tethyan nannoconids indicates high-temperature surface waters and oligotrophic environments. The predominately marly sequence with mixed Tethyan and Boreal faunas and floras in Unit 3 indicates a change in the depositional regime from shallow-water to more open marine conditions. Both the facies and palaeobiologic changes argue for a latest Barremian sea-level rise in the South Carpathian area. A late Barremian transgression prior to the widespread earliest Aptian sea-level rise has been reported previously from other European regions. In the Speeton area of eastern England, the presence of both Tethyan and Boreal ammonites (Rawson, 1994) is attributed to this transgression (Rawson, 1995), which is also inferred in southeast France (ArnaudVanneau and Arnaud, 1990).

537

The recurrence of reef facies (calcarenites and calcilutites with rudist and mollusc fragments) at the base of the Aptian Unit 4 is comparable to that of Unit 2 and indicates restoration of a shallow-marine environment. Since the characters of the fauna and flora are entirely Tethyan, restricted water-circulation in the eastern part of the South Carpathian area is indicated. By contrast, Unit 5 represents another change from facies of a shallow carbonate platform to those a more open-marine environment. This change is also associated with homogenization of the floras and faunas, coincident with the FO of the ammonite Deshayesites, which is considered to be related to the major early Aptian transgressive event. These data agree with those inferring global sea-level changes during the late Barremianeearly Aptian interval (Haq et al., 1987; Fig. 5 herein). 5.2. Palaeobiologic turnover In earliest Aptian, the Early Cretaceous Tethyan and Boreal realms experienced a turnover of planktonic, benthonic and nektonic organisms as attested by the occurrence of cosmopolitan faunas and floras (Mutterlose, 1992, 1998; Erba, 1994; Rawson, 1994; Premoli Silva et al., 1999; Bersezio et al., 2002). Our study indicates that late Barremian deposits in the Romanian study area contain rich, diverse Tethyan microfloras and faunas (Fig. 3). Coeval deposits in northeast Mexico contain mainly benthic foraminifera with common miliolids, lituolids and valvulinids, which are also present in the Romanian section (Figs. 2, 3). However, mixed Tethyan and Boreal ammonites and calcareous nannofloral assemblages in the latest Barremian rock-sequence of the Romanian Carpathians indicate a measure of palaeobiologic homogenization at that time, a phenomenon that only occurs worldwide later in the early Aptian. Our study suggests that the earlier occurrence of this bioevent is probably related to a regional transgression in Romania. The overlapping fossil components from the two realms coincides with a sharp decline of the nannofloral genera Micrantholithus and Braarudosphaera, and an increase in abundance of Watznaueria barnesae. It is also concurrent with a decline in narrow-canal species of Nannoconus and an increase of wide-canal taxa. These fluctuations took place between the FO of Hayesites irregularis and the base of the Aptian (Figs. 3, 4) that we correlated in both sections with the FO of the foraminifer Palorbitolina lenticularis. The Barremian/Aptian boundary is considered to be at the base of Magnetic Chron MO, approximately indicated by the boundary between the Melchiorites sarasini and Deshayesites tuarkyricus ammonite zones (Erba, 1996). Based on correlation with the ‘‘standard’’ ammonite zonation of the West Mediterranean Province, this boundary falls between the Pseudohaploceras waagenoides and Deshayesites oglanlensis ammonite zones (Hoedemaeker and Reboulet et al., 2003), within the nannofossil Chiastozygus litterarius Zone of Sissingh (1977), and within the foraminiferal Globigerinelloides blowi Zone (Premoli Silva and Sliter, 1994; Weiss, 1995). In the section of Gorgo a Cerbara in central Italy (the leading candidate for the GSSP of the Barremian/Aptian boundary)

R. Barraga´n, M.C. Melinte / Cretaceous Research 27 (2006) 529e541

MAIN BIOEVENTS (Romania)

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the chron MO is slightly younger than the FO of Hayesites irregularis, and the planktonic turnover occurs within the G. blowi and C. litterarius zones (Coccioni et al., 1992; Premoli Silva et al., 1999). In both of the sections studied we consider the Barremian/ Aptian boundary to coincide with the FO of Palorbitolina lenticularis within the interval associated with the acme of representatives of Orbitolina. In Romania these events are recognized within the Chiastozygus litterarius Zone, between the FO of Hayesites irregularis (the bioevent that best approximates the Barremian/Aptian boundary according to Erba, 1996, and Bersezio et al., 2002), and the FO of Braarudosphaera hockwoldensis. In Mexico, the event is also defined

by the FO of Pseudocyclammina hedbergi and the LO of Pseudocyclammina litus (Figs. 2, 5). In both regions, the base of the Aptian as defined by the first beds above the orbitolinid acme zone lacks the ammonite genus Deshayesites; indeed the uppermost Barremianelowermost Aptian successions of northeast Mexico are devoid of ammonites up to the uppermost lower Aptian deposits where American ammonites representing the local Dufrenoyia justinae Zone (Fig. 5) become abundant. Furthermore, although Deshayesites occurred in European areas of the Tethys (France, Spain) from the beginning of the Aptian (Aguado et al., 1997; Cecca et al., 1999), the earliest representatives of this taxon in the South Carpathians occur well above the Barremian/Aptian

R. Barraga´n, M.C. Melinte / Cretaceous Research 27 (2006) 529e541

boundary (Fig. 3) and above the FO of Neohibolites spp. in a bioevent that coincided with the onset of deeper-water deposition over the previous carbonate platform and a drastic decrease in the abundance of Nannoconus (Fig. 4), termed the ‘‘nannoconid crisis’’ and first described by Erba (1994) from Italian sections. In contrast to this drastic decline, however, Nannoconus spp. markedly increase in abundance in the bed immediately above the Barremian/Aptian boundary of the Romanian Carpathians. Micrantholithus and Braarudsophaera also increase in abundance, whereas numbers of Boreal taxa, as well as Zygodiscus erectus and Cyclagelosphaera margerelii, decline. These variations indicate a brief restoration of more oligotrophic conditions. Numbers of Nannoconus spp. decline gradually following this initial increase, until the pronounced drop in abundance mentioned above prior to Oceanic Anoxic Event (OAE) 1 in other Tethyan areas (Erba, 1994). We note also that the FO of Deshayesites coincides with a significant increase in abundance of Boreal nannofossils and of Zygodiscus erectus and Cyclagelosphaera margerelii, high fertility (Bischoff and Mutterlose, 1998; Premoli Silva et al., 1999) indicating a return to a eutrophic environment. These characteristics persist in the uppermost part of the lower Aptian sequence studied in Romania, above the FO of Deshayesites. 6. Conclusions Late Barremianeearly Aptian rock sequences in Durango, Mexico, and the Daˆmbovicioara area of the South Carpathians, Romania, have revealed palaeoenvironmental and palaeobiologic turnovers within this time interval. The late Barremian sediments of the Cupido Formation were deposited in a shallow lagoonal environment of a carbonate platform. Lithologic characteristics indicate that the same depositional regime extended into the earliest Aptian. Facies characteristic of the La Pen˜a Formation began to accumulate in the latest early Aptian, and indicate a shift from a shallow carbonate platform to a facies of a more open-marine environment, probably of an outer ramp or slope. This major change probably resulted from a major transgression that led to drowning of the carbonate platform. The palaeobiologic turnover recorded in the section shows a proliferation of planktonic foraminifera in the uppermost lower Aptian, following an acme of benthonic Orbitolina. Similarly, late Barremian sediments of the Daˆmbovicioara Formation in Romania are characteristic of a shallow-carbonate platform. Unlike the Mexican section, however, we identify a sea-level rise in the latest Barremian. This event was succeeded by restoration of a shallow-marine environment in the earliest Aptian. Subsequently, a major change occurred during the early Aptian when reef limestones and calcarenites were completely replaced by marl-dominated sequences containing mixed Tethyan, Boreal and cosmopolitan floras and faunas, probably as a consequence of the global sea-level rise in the early Aptian. Based on the increase abundance of pentalith nannofossils such as Micrantholithus spp. and Braarudosphaera spp., the latter also showing radiation of species, we infer that the ecosystems were unstable prior to the ‘‘nannoconid crisis’’,

539

and the advent of OAE1. Our results are compatible with those previously reported from other Tethyan areas (e.g., in Italy, Bersezio et al., 2002) that have been related to climate changes and perturbation in the carbon cycle (Weissert et al., 1998) linked to the intraplate magmatism in the Pacific Ocean (Erba, 1994; Larson and Erba, 1999). Fluctuations in the nannofloras represented by the ‘‘nannoconid crisis’’ and significant increases in numbers of high-fertility indicators and Boreal species argue for the development of eutrophic environments. In both study areas existing shallow-marine environments of the late Barremian were replaced by deep-marine conditions during the early Aptian. This change was probably caused mainly by worldwide sea-level rise, which allowed further interchange of faunas and floras. Other controlling mechanisms that caused drowning of the carbonate platforms may have been related to tectonic events and regional palaeophysiographic factors; these played a significant role in Mexico during the late Barremianeearly Aptian (Maurrasse, 2003). Acknowledgements We are grateful to two anonymous referees who critically reviewed the paper, providing useful remarks and comments. Field work for this study was partially supported by the Mexican CONACyT-SEP project 40337-F. We also thank Dr. Florentin Maurrasse and Prof. David Batten for linguistic revision and their useful suggestions to improve the manuscript. References Aguado, R., Company, M., Sandoval, J., Tavera, J.M., 1997. Biostratigraphic events at the Barremian/Aptian boundary in the Betic Cordillera, southern Spain. Cretaceous Research 18, 309e329. Arnaud-Vanneau, A., Arnaud, H., 1990. Hauterivian to Lower Aptian carbonate shelf sedimentation and sequence stratigraphy in the Jura and northern Subalpin chains (southern France and Swiss Jura). International Association of Sedimentologists, Special Publication 9, 203e233. Arthur, M.A., Premoli-Silva, I., 1982. Development of wide-spread organic carbon-rich strata in the Mediterranean Tethys. In: Schlanger, S.O., Cita, M.B. (Eds.), Nature and Origin of Cretaceous Carbon-rich Facies. Academic Press, London, pp. 7e54. Avram, E., Melinte, M., 1998. BarremianeAptian boundary in the Daˆmbovicioara area (Romanian Carpathians). Zentralblatt fu¨r Geologie und Pala¨ontologie, Teil I, 11/12, 117e129. Barragan, R., 1999. Sedimentary facies and organic carbon variations in BarremianeAptian sequences of northeastern Mexico. Revista Espan˜ola de Micropaleontologı´a 31, 305e314. Barragan, R., 2001. Sedimentological and paleoecological aspects of the Aptian transgressive event of Sierra del Rosario, Durango, northeast Mexico. Journal of South American Earth Sciences 14, 189e202. Barraga´n-Manzo, R., Dı´az-Otero, C., 2004. Ana´lisis de microfacies y datos micropaleontolo´gicos de la transicio´n BarremianoeAptiano en la Sierra del Rosario, Durango, Me´xico. Revista Mexicana de Ciencias Geolo´gicas 21, 247e259. Barraga´n-Manzo, R., Me´ndez-Franco, A.L., 2005. Towards a standard ammonite zonation for the Aptian (Lower Cretaceous) of northern Mexico. Revista Mexicana de Ciencias Geolo´gicas 22, 39e47. Barron, E.J., 1983. A warm equable Cretaceous: the nature of the problem. Earth-Science Reviews 19, 305e338. Bersezio, R., Erba, E., Gorza, M., Riva, A., 2002. BerriasianeAptian black shales of the Maiolica Formation (Lombardian Basin, Southern Alps,

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